1. Field of the Invention
This invention relates generally to a frost monitoring and alarm system for plant husbandry and more particularly to a system which is used for the prevention of damage to fruit trees and other plants by local weather conditions, namely freezing temperatures.
2. Description of the Prior Art
Frost protection for orchards is a vital part of management in many fruit growing areas. With citrus and stone fruits, in particular, crop damage with resulting economic loss results if the buds, blossom or small fruits are frozen. For years the practice has been to heat these orchards when temperatures drop to the point at which tissue freezing commences. Heating methods have consisted largely of burning fossil fuels. In recent years large fans or wind machines have been used to bring warmer upper air down to tree level when a condition of temperature inversion is present. Whatever the method of frost protection used, fuel of some sort is expended. With fuel shortages and greatly increased cost of fuel, the basic problem of orchard heating has become critical.
The problem presents itself then as to when to start up the frost protection system. When fuel was relatively inexpensive, many orchard owners or growers would "light up" early just to be on the safe side. However, a cold spring with many heating nights can be disastrously expensive. Each successive heating night reduces the grower's potential profit. Additionally, with today's high fuel costs and emphasis on conservation, the grower who unnecessarily operates his heating system, wastes both money and resources. From experience and research, it has been determined approximately what temperature a given fruit variety can stand at a given stage of its development. This might vary from 20.degree. F. to 30.degree. F. during the season. For example, if cherries at a given stage of development will stand 28.degree., the grower will "light up" at some point above 28.degree. F. If the temperature drops below 28.degree. F., he has saved enough of the crop to justify the heating expense. If he lights up at 29.degree. F. and the temperature does not go below that, then he has wasted both fuel and money. In the past, the determination as to when to "light up" was based on hit and miss procedures dictated by monitoring the local weather forecast, by watching orchard thermometers set in the coldest area of the orchard, and by making use of one's own past experience. Frost protection as a result has become probably more of an "art" than a "science" and at best an inexact science.
It should also be noted with some emphasis that an orchard thermometer measures atmospheric temperature and not that of the trees themselves. Since the grower is interested in what is happening to the tree parts, measuring the ambient temperature by placing thermocouples, for example, on the tree bark would provide little more information than an indication of air temperature. Such measurements do not indicate the most pertinent piece of vital information and that is at what point does the tree tissue actually freeze.
It has been previously observed that tree tissue exhibits the phenomenon that its electrical resistance increases dramatically at the temperature at which freezing of the tissue occurs. This has been documented in a publication entitled "A Note on Electrical Resistance Measurements in Acer Saccharum" by D. S. Fensom, which appeared in the Canadian Journal of Botany, Volume 38, 1960, at pages 263 and 264 and in an article entitled "Freezing Patterns in Twigs of Evergreen Azalea", by G. P. Lumis, et al. which appeared in the Journal of the American Society of Horticultural Sciences, 99 (6), 1974 at pages 564-567. It is upon this observed characteristic that the present invention is based, as will be set forth subsequently.
A preliminary/patentability search has been conducted and the following references were noted and are hereby made of record:
U.S. Pat. Nos: 2,419,266; 2,717,957; 3,046,537; 3,182,914; 3,298,191; 3,463,396; 3,553,481; 3,594,775; 3,706,981; 3,777,976; 3,778,798; 3,891,979.
None of the above references is believed to be particularly pertinent to the subject invention. Typically, U.S. Pat. No. 3,298,191 discloses a solid state control system for the so-called ice bank on refrigeration coils with the operation of the system being in the utilization of a change of resistance when water changes to ice and the generation of a control signal. Accordingly in some other examples the irrigation systems are turned off and on in response to changes in variation in resistance of sub-surface sensors which are responsive to moisture content of the soil. Reference is accordingly made to U.S. Pat. Nos. 3,777,976; 3,182,914; 3,553,481; and 3,463,396.